1. Field of the Present Invention
The present invention relates to a method of charging a storage battery comprising executing an n(nxe2x89xa72)-stage constant current charging process charging the battery with a constant current while monitoring a terminal voltage of the battery, the current decreasing stepwise nxe2x88x921 times every time the terminal voltage coincides with a current switching voltage, in particular, the method of charging which can charge the battery very quickly and prevent a reduction in the life of the battery.
2. Discussion of the Related Art
An n(nxe2x89xa72)-stage constant current charging process as disclosed in, for example, JP-A-61/26438, is suitable for charging a storage battery quickly. Therefore, the process is applied for various kinds of storage batteries, preferably lead storage batteries, in particular the lead storage batteries for electric vehicles. FIG. 10 shows a change of a current I for charging and a terminal voltage V of the battery with time in the four-stage constant current process. In this process, the currents I1, I2, I3, I4 of respective stages are set so that the current I for charging decreases stepwise as the stage progresses. At first, the current I1 in the first stage is applied to the battery while the terminal voltage V is monitored. The current I1 is switched to the current I2 in the second stage when the terminal voltage V coincides with a current switching voltage Vs. The current I for charging is similarly switched twice more every time the terminal voltage V coincides with the current switching voltage Vs. The process stops when the terminal voltage V in the fourth stage reaches a final charge voltage Vf of the battery.
In the method of charging the battery with the n-stage constant current process, the current for charging the storage battery is frequently restricted to a relatively low current in consideration of temperature increase and deterioration of the battery. For example, the constant current I1, I2, I3, I4 in each stage is that corresponding to, respectively, 0.2C (C: a rated capacity of the battery), 0.1C, 0.05C and 0.025C in the four-stage constant current charging process as shown in FIG. 10.
Therefore, the storage battery takes a long time to be charged with the n-stage constant current process when the battery is deeply discharged. It is desirable that the battery is charged within a time during which a discount commercial electric power rate is applied. However, it is often impossible to fully charge a severely discharged battery with the n-stage constant current charging process within the desirable time.
One of the measures to shorten a charging time is to increase the constant current I1xe2x80x2 in the first stage as shown with an imaginary line of FIG. 10. However, the charging time does not notably decrease in total, because the charging time in the second stage gets longer though the terminal voltage in the first stage coincides with the current switching voltage Vs faster. Furthermore, long charging time with large current frequently leads to deterioration of the battery.
It is therefore an object of the present invention to provide a method of charging a storage battery which makes it possible to charge the battery very quickly.
It is another object of the present invention to provide a method of charging the battery which can prevent a reduction in the life of the battery.
It is a further object of the present invention to provide a method of charging the battery which can level off commercial electric power consumption per hour.
The above objects are achieved based on a method of charging a storage battery, preferably a lead storage battery, in particular the lead storage battery for an electric vehicle, comprising executing an n(nxe2x89xa72)-stage constant current charging process charging the battery with a constant current while monitoring a terminal voltage of the battery, the current decreasing stepwise nxe2x88x921 times every time the terminal voltage coincides with a current switching voltage.
The method of the present invention further comprises measuring a depth of discharge of the battery immediately before charging and executing an additional charging process before the n-stage constant current charging process when the depth of discharge is deeper than a reference depth of discharge, the additional charging process charging the battery with an additional constant current larger than an initial current in the n-stage constant current charging process and switching the additional current to the initial current when the terminal voltage coincides with the current switching voltage.
The reference depth of discharge is predetermined so that the storage battery having the reference depth of discharge is fully charged with the n-stage constant current charging process at the end of a time permitted for charging. The time permitted for charging depends on the kind and the usage of the battery, and is preferably the time during which a discount commercial power rate is applied.
Time required for charging is largely shortened by the additional charging process according to the present invention. Therefore, it is possible to fully charge a severely discharged battery within the time permitted for charging, for example, within the time during which the discount commercial power rate is applied.
In the method of the present invention, the additional current in the additional charging process is preferably set so that the additional current increases stepwise or continuously as the depth of discharge of the battery increases. Temperature increase and deterioration of the battery during the additional process can be controlled by this method.
The method of the present invention can further comprise selecting one of a normal charging mode and an additional charging mode. In this method, the additional charging process is executed only when the additional charging mode is selected and the depth of discharge is deeper than the reference depth of discharge. That is, the additional charging process is carried out as required.
The method of the present invention can further comprise detecting a starting time of charging. In this method, the additional charging process is executed only when the starting time of charging is within a time predetermined for charging and the depth of discharge is deeper than the reference depth of discharge. The additional charging process can be executed only when the starting time of charging is within a time predetermined for each of the battery so as to level off commercial electric power consumption per hour. This method advantageously contributes toward leveling off the commercial electric power consumption per hour. The additional charging process can be executed only when the starting time of charging is within the time during which the discount commercial electric power rate is applied. The method leads to a low cost charging as well as the leveling off the electric power consumption per hour.
The method can further comprise detecting a temperature of the storage battery immediately before charging. In this method, the additional charging process is executed only when the temperature of the battery is lower than a temperature at which a reduction in the life of the battery begins to accelerate and the depth of discharge is deeper than the reference depth of discharge. Temperature increase of the battery, which leads to thermal runaway and deterioration of the battery, is prevented by this method. The current of the additional charging process is preferably set so that the current of the additional charging process decreases stepwise or continuously as the temperature of the battery increases in this method. The temperature increase and deterioration of the battery during the additional process can be minimized by the method.